Back in the early 80s, when my husband and I started our publishing company, we got 100 ISBNs from Bowker. Free! I still have the original logbook, almost half used up. I usually log in to Bowker and update when we publish something, but it wasn't until today that I noticed an oddity... I went to bookow.com's barcode generator as I'm finishing a book cover and I hadn't yet discovered that CS wouldn't use it anyway. I typed in the ISBN I had chosen from our logbook and Bookow wouldn't take it. I tried a few times, no luck

So I logged into my Bowker account and discovered that the numbers listed are a bit different from what our logbook has. The number I had chosen was our usual start, 0-916289-, and then it was 43-5. No such number existed online. So I printed out the new logbook and this time when I went to bookow, it accepted one of the numbers from there.

So that's the oddity.

Questions:
[1] Any idea why Bowker changed our numbers on us? I have no idea when in the last 40 years they did this... my only guess is that it might have to do with check digits.

[2] Back then I also got a SAN which I haven't used in ages. Are people still using them? Should I?

Your original logbook probably contains ISBN-10 numbers, which are now deprecated in favor of the ISBN-13 system (they ran out of numbers so had to make them longer - more digits). If you use the 13-digit version of your remaining numbers, you shouldn't have any problems.

Let me add a bit to Sarah's answer, at one level, the 10 digit ISBN that you have is the same as the 13 digit ISBN you can use by converting the 10 digit number (you can find a converter at: https://www.isbn.org/ISBN_converter).

Here is a 10 digit ISBN from a book I picked out from those near at hand:
0-8230-1877-6
I convert that 10 digit number to get:
978-0-8230-1877-2

The original number was 0-8230-1877 to which a check number was appended, in this case 6.

To create the 13 digit number 978 prefix is added to the original 9 digits: 978-0-8230-1877, to which a new check number was appended, in this case 2.